Where is the geometry nodes modifier?
If you are looking for it in with other modifiers as subdivision and displace, i dont think you can find it there.
Select the object you want it to be applied and in the editor type (where you go to switch between 3d view and shader editor and all) you will find it there just in between shader and texture node editor options.
And its only available in 2.92 and above. I think . Not sure if 2.91 has it.